›Introduction to DNA and Genealogy
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 ›Meiosis and Crossover


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* Meiosis - basics
* What is a Centimorgan?

Each person is created from a 50:50 blend of their parents Chromosomal DNA - half your chromosomes are from one parent and half of your chromosomes from your other parent. Each person normally has 46 chromosomes, and they were created from 1 egg and 1 sperm, each with 23 chromosomes. There are 22 pairs of matching "Autosomal" chromosomes normally, and a final pair which determine a persons gender. In women, the final pair consist of two matching X chromosomes, and in Men the final pair are a mismatched single X chromosome and a single Y chromosome.

The process in which the genetic material from each parent is halved to form the DNA contribution in the egg or sperm is called Meiosis. A simplified version of this process is as below:

This simplified diagram only shows 1 of the 23 chromosome pairs, - Chromosome 8. It shows the meiosis events that occurred in the formation of the two gametes that formed "Daniel O'Neill".
In reality, the same process occured for all of Samanthas 23 pairs of Chromosomes, and for 22 of Jacks 23 pairs of Chromosomes. Jacks final pair of chromosomes did also undergo meiosis, - but crossover generally does not occur between the mismatched pair. A very very small degree of crossover can occur between a X and Y pair, - but the amount is so small that it can be considered negligible (the Y-DNA markers we use for genealogy purposes are in areas of the Y chromosome that crossover does not occur in except for exceptionally rare instances).

(click here to see a more comprehensive explanation of the process of meiosis).

Note how the process of meiosis is slightly different for egg production compared to sperm production. For sperm production, each meiotic event results in 4 gametes (each of which has a chance to fertilise the egg), but for egg production, only one gamete is produced per meiotic event. See the table below for a direct contrast between egg and sperm production.

Meiosis during Sperm production (Spermatogenesis)
Meiosis during Egg production
Meiosis begins
70 days before Daniels conception
June 29 2013
16000 days before Daniels conception (approx 3 months before Samanthas Birth)
October 21 1968
Chromosomes usually just have a single "arm". It is only when cells are in the process of dividing that they have two arms. In this stage, the Chromosome arms double.
40-70 days before Daniels conception
June/July 2013
15910-1600 days before Daniels Conception
November/December 1968
Crossover begins. Two arms, one each of the two matching chromosomes line up together. Special "bridges" form at different points, joining the two chromosomes together. They then swap stretches of DNA between the "bridges".
40-70 days before Daniels conception
June/July 2013
15910-1600 days before Daniels Conception
November/December 1968
Crossover completed. The two arms from the matching chromosomes move apart. Each arm now has one arm identical to how it was before crossover, and one arm that is now a blend of both matching chromosomes.
About 40 days before Daniels conception This stage enters a kind of "stasis" from just before birth until after puberty.
December 1968 - July 2013
Cell splits into two, one chromosome from each pair goes into one of the daughter cells, and the other chromosome from each pair goes into the other "daughter" cell. Each chromosome still has two arms. This is the first "throw of the coin".

Both "daughter" Cells progress to the next stage.


2-40 days before Daniels conception
July 2013

Only one of the "daughter" cells progress to the next stage. When the cell divided, most of the cytoplasm (cell fluid) went into just one of the two cells rather than being evenly divided. The cell with hardly any cytoplasm is called "the first polar body" and is "discarded"

Completed August 2013

The cell splits into two again. One arm from each chromosome goes into each of the two daughter cells. This is the second "throw of the coin".

All "daughter" Cells progress to the next stage, making 4 sperm from each Meiosis event.


2 days before Daniels conception.
August 13 2013

Only one of the "daughter" cells progress to the next stage, making just one egg from each Meiosis event. As with the previous stage, one of the two daughter cells receiveds less cytoplasm and is "discarded".

1 day before Daniels conception
August 14 2013

Gametes meet - 266 days before Daniel O'Neill was born. August 15 2013
Fertilisation - 266 days before Daniel O'Neill was born. August 15 2013

Grandparent/Child inheritance diagrams

On the main webpage we asked the question: So,.. - how many DNA "coin tosses" are there??... We have 23 pairs of chromosomes, but there are there are billions of SNP's. So,.. is it 23 coin tosses? or is it billions of coin tosses?
... and the answer given was- it is neither!.

From the simplified diagram above, we can see that .. instead of being a single coin toss for each chromosome,.. it's kind of like a random scambling followed by two consecutive coin tosses. The first coin toss being grandmas v's grandpas Chromosome, and the second coin toss being the unblended arm v's the blended arm.

In theory, - we'd expect 50% of inherited chromosomes to be a blend of both grandparents, and 50% of chromosomes to be from one grandparent or the other. In practice though,.. from the diagrams below it looks like that more than 50% of chromosomes inherited from a parent are a blend of those from the grandparents, rather than 50% of chromosomes identical to a grandparents and 50% a blend.
Why is this so?.... (I don't know the reason, and I'm curious to find out the answer....).

"A unit of recombination frequency , implying distance along a chromosome . One centimorgan (cM) is equivalent to a 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossover per generation. In humans, 1 cM is equivalent to (on average) 1 million base pairs"

What is a Centimorgan? - It's almost like asking "how long is a piece of string",.. it depends....

The official definition is:
"A unit of recombination frequency, implying distance along a Chromosome"

A centimorgan is on average 1 million base pairs long, but the actual length of a centimorgan can vary depending on the specific Chromosome, and specific location on that chromosome

So, what does that mean in plain English?.... Using the piece of string analogy, -
"a piece of string is on average 1 metre long, but the actual length of a specific piece of string depends on the ball of string it is from, and where in that ball of string it is"

eg. a centimorgan can be a different length closer to the end of a Chromosome compared to the centromere

What determines the length of a centimorgan is: - how likely is it that a crossover point has occurred in that region of the chromosome?

You could say that the length of a piece of string is smaller in a region where crossover happens alot, and the length of the string is longer in a region where crossover doesn't happen quite so much... or a centimorgan is less than 1 million bases in a region where crossover occurs more frequently, and a centimorgan is more than 1 million bases in a region where crossover occurs less frequently than average.

Why is Centimorgan length important for genealogy?

If you have a match of (for example) 500 base pairs with a potential cousin, that match is more significant if the exact location of the matching DNA is on a place on a chromosome where recombination is more likely to happen (ie. the bits of DNA from your ancestors are more likely to be mixed up) than if the location is of a low centimorgan value (ie. the bits of DNA from your ancestors are less likely to be mixed up).

Help - If you are a grandparent that has tested with 23andMe (or have tested with family finder at FTDNA) and your grandchild has tested as well (or you are a grandchild whose grandparent(s) has/have tested also), then it would be very helpful if you could add your genome comparison to my collection.

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